DC/DC converters are electronic devices that use switching components, such as field effect transistors (FETs) to transform voltage from one level to another. Typically, the output voltage is regulated and protected against short circuits. In many cases, the input and output potentials are galvanically isolated from each other.
A preferred semiconductor device for power switching in a DC/DC converter is the insulated gate FET (Field Effect Transistor) because of its high power gain. FETs used for power switching use are usually enhancement mode types. This means that they are normally non-conducting. When a gate voltage above a threshold is applied, the FET becomes conducting. FETs are available in two gate polarities; N channel and P channel.
In an FET, current flows along a semiconductor path called the channel. At one end of the channel, there is a source electrode, and at the other end, a drain electrode. The physical diameter of the channel is fixed, but its effective electrical diameter is changed by applying voltage to a gate electrode. The conductivity of the FET depends, at any given time, on the electrical diameter of the channel. A small change in gate voltage can cause a large variation in current from the source to the drain. In this way, the FET switches current on or off.
Typically, FETs used for power switching are enhancement mode types, that is, they are normally non-conducting. When a gate voltage above a certain threshold is applied, the FET becomes conducting. Such FETs are used to control current flow and are available in two gate polarities; N channel and P channel.
Among many applications, DC/DC converters are used in spacecraft, satellites and in high energy physics instrumentation where they are subjected to many forms of radiation damage. When electrical components are exposed to radiation, they behave differently. For example, when an N channel FET is exposed to relatively low radiation levels, the gate threshold voltage ultimately falls close to zero. In this condition, the FET conducts current with little or no applied gate voltage. In other words, the FET is uncontrollable because the current running through the channel cannot be easily shut off.
DC/DC converters and power switching circuits designed for general purpose use are typically constructed with N channel FETs because, for any given die size transistor, the N channel FET has a lower on resistance than a correspondingly sized P channel FET.
In general, a DC/DC converter or switching regulator includes a power chopping stage which converts the DC input power to a periodically pulsating DC waveform. This stage is followed by a filtering stage where the periodically pulsating DC waveform is converted back to a DC level. A transformer may be interposed between the power chopping stage and the filtering stage to provide input to output isolation.
The DC/DC converter or switching regulator also includes repetitive pulse drive circuitry which controls the operation of the power chopping stage so as to achieve the desired power output.
To use electrical components in high radiation environments, they are often designed to withstand the damage caused by radiation. Present art for radiation hardened DC/DC converters use specially designed radiation hardened N channel FETs for the power chopping stage. The radiation hardening process usually involves removing or adding some specific element or ions to the materials used for making the components. Being radiation hardened the gate threshold voltage experiences minimal change after exposure to radiation. One method for chemically radiation hardening DC/DC converters is disclosed in U.S. Pat. No. 3,836,836 to Cowett, Jr. (Cowett).
The principal benefit of radiation hardened N channel FETs is that the gate threshold voltage doesn't change significantly with radiation exposure. The DC/DC converter therefore functions despite the accumulated radiation dose. Additionally, the downside of these specially designed radiation hardened N channel FETs is that they (1) have a sole source of supply, (2) are expensive, (3) have long lead times and (4) have limited availability. In turn, this affects the market for radiation tolerant DC/DC converter circuits incorporating this type of FET with higher prices, longer delivery times and limited availability.